1. Field of the Invention
The present invention relates to the field of electric machines. It relates to a winding bar for the high-voltage winding, in particular stator winding, of an electric machine, comprising a plurality of conductors which are arranged above and/or next to one another, and a conductor bundle with a rectangular cross section, the conductor bundle being surrounded outside by an insulation.
In this case, the conductors can be arranged electrically in parallel (bar winding) or be interconnected in series (coil winding). However, in normal operation the voltage between the conductors is substantially smaller than that across the bar insulation.
The invention also relates to a method for producing such a winding bar.
2. Discussion of Background
Winding bars such as are used, for example, in stators of rotating electric machines frequently have the cross section represented by way of example in FIG. 1. The winding bar 10, which is inserted into a slot 11, provided for the purpose, in the stator laminated core 12, comprises a bundle of individual conductors 13 which are arranged running in parallel above and/or next to one another. The conductor bundle, which generally exhibits a rectangular cross section with edges 15, is surrounded on all sides by an insulation 14. As a result of the shape, the electric field at the edges 15 is stronger than on the flat sides of the winding bar 10. Consequently the edge region is particularly susceptible to electric breakdown or electric long-term failure.
In order to achieve the best possible filling of the slot with conductor material, and the best possible transmission of heat via the bar insulation, an attempt is made to achieve the thinnest possible insulation, at least on the flat sides of the bar, which dominate in terms of area. The conventional production of the insulation 14 in the case of winding bars with a rectangular cross section is described, for example, in xe2x80x9cSequenz: Herstellung der Wicklungen elektrischer Maschinenxe2x80x9d [xe2x80x9cSequence: Production of windings of electric machinesxe2x80x9d], Springer-Verlag 1973, pages 128-129. According to this, strips of mica paper, which is coated with glass fabric on a substrate for the purpose of increasing the tensile strength and tear resistance, are wound in the form of layers around the bar or conductor bundle, subsequently impregnated with synthetic resin, molded-in and cured at raised temperatures. The thickness (d1 in FIG. 1) of the insulation 14 is approximately the same on all the flat sides of the winding bar 10 in this method. At the edges 15, it theoretically exhibits the same thickness d1 (see the enlarged partial section in FIG. 2), but becomes smaller in practice because of the locally increased contact pressure (small supporting surface in the edge region) which is produced when the strip is wound around the conductor bundle at a constant rate with a constant winding tension. According to the formula for coaxial cylinders, the maximum electric field at the edges 15 can be specified as:       E    max    =            U                        r1          ·          ln                ⁢                  xe2x80x83                ⁢                  r2          r1                      =          U                        r1          ·          ln                ⁢                  xe2x80x83                ⁢                              r1            +                          d              *                                r1                    
In this case, (in accordance with FIG. 2), U is the on-load voltage, r1 the inner radius of curvature of the insulation 14, r2 the outer radius of curvature of the insulation 14, and d*the thickness (mostly reduced with respect to d1) of the insulation 14 in the region of the edges 15. It is clear from this that the electric field which in the case of radii r1xe2x89xa63 mm which are technically easy to realize is in any case already distinctly stronger at the edge 15 than in the region of the flat sides, will once again increase as a consequence of the reduced insulation thickness d*, as results in the case of many production methods.
The effects of the increased field strength can be considerable, particularly in the case of continuous electric loading, since the failure rate txe2x88x921 increases with the electric field in a strongly superlinear fashion. In rough terms, a law of exponents holds between the lifetime t (in h) and electric field E (in kV/mm) in accordance with             t              t        0              =          K      ·                        [                      E                          E              0                                ]                          -          n                      ,
t0=1 h and E0=1 kV/mm. For a service life exponent n=8 (this is a frequent value in the case of insulating materials for rotating electric machines in accordance with the prior art), this means, for example, that an increase in the field strength by 20% reduces the service life to less than xc2xc, but conversely that reducing the field by 20% increases the service life by a factor of approximately 6.
Lowering the edge field strength could now be achieved, for example, by going over from an insulation 14 with constant thickness d1 (=d*), as is shown in FIG. 1 and FIG. 2, to an insulation with an angular outer contour (r2=0). This assumption, carrying out a computer model calculation based on the finite elements method for a value of d1=2.5 mm and r1=2.5 mm, produces a reduction in the maximum field strength Emax in the edge region of 11%, which corresponds to a computational prolongation of service life by the factor 2.5 in the case of a service life exponent of 8. This factor increases disproportionately with a higher service life exponent (for example factor 4 for n=12).
Accordingly, one object of the invention is to provide a novel winding bar in which a distinct reduction in the maximum field strength in the edge region is achieved with simple means without the need to increase the thickness of the insulation in the region of the flat sides, as well as to specify a method for producing said bar.
The object is achieved in the case of a winding bar of the type mentioned at the beginning by virtue of the fact that the thickness of the insulation on the edges of the winding bar is greater than the thickness of the insulation on the flat sides of the winding bar. The enlargement of the insulation thickness in the edge region produces a correction of the field lines of the electric field, which leads to the desired reduction in the edge field strength.
A first preferred embodiment of the winding bar according to the invention is defined by virtue of the fact that the insulation on the edges of the winding bar has a curvature with an inner curvature contour and an outer radius of curvature, and that the outer radius of curvature is smaller than the sum of the equivalent inner radius of curvature of the inner curvature contour and the thickness of the insulation on the flat sides of the winding bar. Maintaining an outer radius of curvature differing from zero produces a uniform change, favorable for the field distribution and for the mechanical stability, of the thickness in the edge region.
In principle, such a shaping of the edge regions can be achieved with the aid of a wound insulation when thermoplastic strips are used for the insulation. However, the shaping becomes particularly simple when, in accordance with a second preferred embodiment, the insulation consists of a thermoplastic polymer in which filler particles made from an insulant are distributed. In a preferred development of this, polyetherether ketone (PEEK) is used as the thermoplastic polymer, and mica platelets are used as the filler particles. Instead of PEEK it is also possible to make successful use of other substances such as, for example, polysulfone (PSU) or polyether sulfone (PES).
In accordance with a further preferred embodiment of the invention, the special shaping, used to reduce the edge field strength, of the insulation in the edge region is also extended to the conductors of the conductor bundle surrounded by the insulation in such a way that at least the conductors arranged in the region of the edges respectively comprise a bundle of individual insulated, in particular stranded wires. It hereby becomes possible to use compression molding to bring the conductors themselves into a shape which leads to a further increase in the insulation thickness in the edge region. In this case, the wires can be formed, for example, as round wires, but can also be square (in the manner of miniature ROEBEL bars).
In a preferred development of this embodiment, for the purpose of achieving a smooth outer contour the bundles of the wires are impregnated with a semiconducting binding compound, and for the purpose of coupling to the semiconducting binding compound at least one wire in the bundle of wires is left or made bare. It is hereby possible to minimize an unfavorable influence of the wire bundles on the field distribution. Over and above the smoothing of the outer contour of an individual conductor, the outer contour of the entire conductor bundle can be smoothed when, in accordance with a further embodiment, the conductors of the winding bar are spaced apart from one another by interspaces, and the interspaces are filled with a semiconducting bridging compound which preferably exhibits (in the bar direction) a voltage-limiting behavior which is nonlinear, at least in sections.
The method according to the invention for producing a winding bar, in the case of which winding bar the insulation consists essentially of a thermoplastic material, is defined by virtue of the fact that after the application of the insulation the winding bar is subjected to a calibration process in which there is impressed on the deformable material of the insulation a prescribed edge contour which leads to an increased thickness of the insulation in the region of the edges. The decoupling of the processes of application and shaping renders it possible for the two process steps to be optimized separately and carried out continuously at the same time. The insulation can be applied in this case in particularly uniform layers, which can be effectively shaped, when, in accordance with a first preferred embodiment of the method according to the invention, the insulation is applied by means of a method from the range of the powder-coating methods, in particular spray sintering or thermal spraying, and extrusion methods.
The method according to the invention can be carried out in a particularly quick and precise fashion when, in accordance with a further preferred embodiment, for the purpose of carrying out the calibration process, calibration rolls which are arranged parallel to the middle planes of the winding bar and at a fixed spacing therefrom are moved relative to the winding bar over the surfaces thereof, and within the calibration process a prescribed radius of curvature is impressed on the edges of the winding bar by means of additional edge rollers.
Further embodiments follow from the dependent.